util: avoid a name-error warning in the `mmapread` exception handler

Newly caught by pytype 2021-09-09.

    File "/mnt/c/Users/Matt/hg/mercurial/util.py", line 458, in mmapread: Name 'fd' is not defined [name-error]

Differential Revision: https://phab.mercurial-scm.org/D11470

mercurial/util.py

@@ -449,8 +449,8 b' def mmapread(fp, size=None):'
         return b''
     elif size is None:
         size = 0
+    fd = getattr(fp, 'fileno', lambda: fp)()
     try:
-        fd = getattr(fp, 'fileno', lambda: fp)()
         return mmap.mmap(fd, size, access=mmap.ACCESS_READ)
     except ValueError:
         # Empty files cannot be mmapped, but mmapread should still work.  Check